Magnetic domain propagation sheet



United States Patent US. Cl. 340174 8 Claims ABSTRACT OF THE DISCLOSURE Magnetic devices including sheets of magnetic materials in which single Wall domains are moved may be operated in two different modes. One of those modes is termed the bias-dominated mode wherein the domains retain a stable shape during operation. Conditions for optimizing this mode of operation are described.

FIELD OF THE INVENTION This invention relates to domain propagation devices and more particularly to the propagation of single Wall domains in sheets of magnetic material having a preferred direction of magnetization out of the plane of the sheet.

BACKGROUND OF THE INVENTION Copending application Ser. No. 579,931, filed Sept. 16, 1966 (now Patent No. 3,460,116), for A. H. Bobeck. U. F. Gianola, R. C. Sherwood, and W. Shockley discloses a two-dimensional shift register in which single wall domains are moved controllably along transverse axes in a sheet of orthoferrite.

Such devices are operable in each of two distinct modes which are actually two limiting (extremal) modes. The first mode is one in which the single Wall domain retains any shape to which propagation fields drive it. This mode is called the coercivity dominated mode because the coercivity is sufficiently high to retain the domain shape in the absence of distorting fields. The second mode is one in which the domain regains a stable geometry upon the removal of the distorting fields. The secozd mode is termed a bias-dominated mode because in practical arrangements a bias, of a polarity to collapse single Wall domains, is applied to the sheet in which the domains are moved.

The latter mode of operation has been found to permit particular types of logic operations and is amenable to simple drive geometry. These attributes are discussed in copending application Ser. No. 657,877, filed Aug. 2, 1967 for A. H. Bobeck, H. E. D. Scovil, and W. Shockley, and in copending application Ser. No. 644,351, filed June 7, 1967 for A. H. Bobeck and R. F. Fischer.

SUMMARY OF THE INVENTION It has been found that the thickness of a sheet of magnetic material may be specified to provide optimum operation in the bias-dominated mode and that that thickness may be specified in a generalized manner as a function of a characteristic of the material used for the sheet.

Accordingly, a feature of this invention is a domain propagation device comprising a sheet of material having a prescribed thickness.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic representation of an arrangement in accordance with this invention; and

FIGS. 2 and 3 are graphs showing the coercivity and bias ranges for optimum operation of the arrangement of FIG. 1.

DETAILED DESCRIPTION FIG. 1 shows a memory arrangement in accordance with this invention. The arrangement comprises a magnetic sheet 11 in which single Wall domains are moved from input to output positions.

Representative conductors 12 and 13 are shown associated with input and output positions in sheet 11, respectively. Conductors 12 and 13 are shown connected to input pulse source 14 and to utilization circuit 15 respectively.

Conductors P1, P2, and P3 are coupled to sheet 11 in a manner (not shown) to move single wall domains from input to output positions when pulsed in a three-phase manner. Conductors P1, P2, and P3 are connected to a propagation pulse source 16 to this end.

Sources 14 and 16 and circuit 15 are connected to a control circuit 17 by means of conductors 18, 19, and 20, respectively. The various sources and circuits may be any such elements capable of operating in accordance with this invention.

The operation of the arrangement of FIG. 1 is essentially as described in the aforementioned copending applications. We are concerned here primarily with optimizing that operation. Accordingly, the operation is not described again herein. Sufiice it to say that single Wall domains are introduced at input positions and detected at output positions to which they are moved controllably by consecutively offset propagation fields.

The particular mode of operation of concern here is the mode in which those domains change size and shape only negligibly during operation.

Radial and elliptical changes in domain shape are of prime concern. An unconstrained domain, in a material with low coercivity, changes its configuration spontaneously, for all practical purposes, by expanding or contracting symmetrically (radially) or by distorting into an elliptical shape and then running out into a strip. When driven out of shape by applied fields, the components of the field tending to produce radial and elliptical distortions are least vigorously resisted and so only these types of distortion need be considered in order to determine the conditions for resisting changes of domain shape. The term low coercivity indicates that the coercivity of the sheet is sufficiently low such that it does not maintain a domain in any shape to which the domain may be distorted by distorting fields and is defined more specifically for optimum operation in accordance with this invention hereinafter.

A criterion for optimum operation is that the conditions are such that any distortion in the shape of the domain is resisted most vigorously. One of the conditions is satisfied when the thickness of sheet 11 of FIG. 1 is chosen such that it is essentially equal to a material length characteristic of the material of the sheet. The material length is not to be confused with the geometric length of the sheet but rather is a property of the material. The material length is defined specifically in terms of the domain diameter D at which the domain collapses in response to increasing bias field. For a circular domain in an infinite sheet of thickness h, the material length D. 1 o 2 Wilma/WE mmddfl] where K and E are standard complete elliptic functions.

1r2 EMU L 1m sin 0 d0 The collapse diameter may be observed experimentally. Consequently, the material length can be determined by calculation and the optimum thickness for a sample magnetic sheet can be specified.

A further condition is satisfied by a range of values for the coercive force H of the material of the sheet. FIG. 2 is a graph from which the range of coercive force values may be determined. The graph has an ordinate in terms of percentage distortion AD/D of a domain diameter D, normalized to the normalized coercive force H ]M I, and an abscissa in terms of sheet thickness h normalized to material length L where lMl is the saturation magnetization in mks units. For a selected value of 1.0 on the abscissa, a vertical line may be drawn intersecting each of two curves shown. The top curve defines the set of points above which the bias is such that in the absence of coercivity a representative domain becomes elliptically unstable. The bottom curve defines the set of points below which the bias is such that in the absence of coercivity the domain becomes radially unstable.

The ordinate points corresponding to the top and bottom intersections are about 10 and 2.5 respectively. We then have AD H for the top intersection and for the bottom. If we select some permissible distortion requirement an D 10 or 3 AD; 1 Tf 10 then H /1 |M[ to satisfy the radial requirement when there is no elliptical stability and H |M| to satisfy the elliptical requirement when there is no radial stability. When both expressions are satisfied simultaneously as specified by some requirement AD (-const.)AD then the requirement is somewhat more stringent. Specifically, for a given condition of operation const.

T, which is the sheet thickness normalized to the material length. For a sheet thickness T of about 1.0, it may be determined from the graph that the bias field may vary from about .185 to .265. For different values of bias H, at any particular sheet thickness T, the domain diameter assumes a corresponding value. The bias is applied to sheet 11 by any convenient field generating means, such as a permanent magnet, as represented by block 25 in FIG. 1.

When a sheet of magnetic material has a proper thickness in accordance with this invention many advantages are provided. First, packing density may be relatively high because domains are smallest and because only negligible changes in domain size during operation need be allowed. Accordingly, the propagation drive geometry may be correspondingly more closely spaced and small. In addition, operating margins are relatively high, for example, because variations in coercivity are more easily tolerated and the conditions between uncontrolled domain expansion and uncontrolled domain collapse enjoy a relatively wide spread in values.

Importantly, design criteria for an entire device may now be specified theoretically rather than empirically.

An example illustrates that the proper choice of sheet thickness permits a significant advantage in domain size and thus packing density. A choice of thickness h of, for example, h=L provides a domain diameter of 2L A choice of thickness h of, for example, h= A L provides a domain diameter of 8L What has been described is considered merely illustrative of the principles of this invention and other and varied modifications may be made therein by one skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A combination comprising a sheet of magnetic material having a preferred direction of magnetization out of the plane of said sheet and having a thickness h, said sheet having a characteristic material length L means for providing single wall domains in said sheet, and means for controllably moving single wall domains in said sheet, said combination being characterized in that h/L is essentially equal to one.

2. A combination in accordance with claim 1 including means for providing in said sheet a substantially uniform bias field of a polarity tending to collapse said domains.

3. A combination in accordance with claim 2 wherein said bias field is in a range of from .185 to .265 of the saturation magnetization of said magnetic material.

4. A combination in accordance with claim 2 wherein said material has a low coercivity.

5. A combination in accordance with claim 4 wherein said material has a coercivity 6. A combination in accordance with claim 4 wherein said material has a coercivity in the range of less than about to of the saturation magnetization.

7. A sheet of magnetic material having a preferred direction of magnetization out of the plane of the sheet and having a thickness h, said material having a characteristic material length L Where h/L is essentially equal to one.

8. A sheet of magnetic material in accordance with claim 7 having a coercivity in the range of less than to M of the saturation magnetization of said sheet.

References Cited UNITED STATES PATENTS 2,984,825 5/1961 Fuller et al 340-174 STANLEY M. URYNOWICZ, JR., Primary Examiner 

